Devices are known for measuring the speed of a helicopter in the above condition, but these devices present various drawbacks. Probes generally used for measuring dynamic pressure due to the movement of the helicopter with reference to the air, and for comparing this dynamic pressure with the static ambient air, are usually not very accurate at the low dynamic pressures corresponding to the low velocities that are to be measured. Besides, unless complex facilities are used that are prohibitory outside a specialized test center, such aerodynamic pressure probes do not make it possible to take into account an orientation velocity of any kind, especially backward or sideways. This wide orientation of movement with reference to the air is precisely one of the essential peculiarities of the helicopter.
To resolve the problems that have been indicated, French patent 1,567,128 in the name of the inventor has already proposed installation below the rotor in the jet flow thereof, a system of two vanes with their axes of articulation substantially horizontal, and oriented respectively along the longitudinal axis and along the transverse axis of the helicopter. This system gives general satisfaction but has some drawbacks and limitations, especially for standardized helicopters intended to be operated with the minimum of constraints in maneuvering and maintenance. In fact, the external disposition of the vanes and their fragility impose precautions and limitations in use, and frequent burdensome checks. Moveover there has to be readjustment of the vanes for each centering value of the helicopter corresponding to a different flight attitude. Besides, since indications furnished by the vanes lack precision beyond the rather homogeneous central part of the rotor jet flow, practical use of this system is limited to low air velocities.
The purpose of the invention is to alleviate these difficulties and limitations, making possible a device for measuring helicopter air speed, especially close to the hovering mode, such that it will be independent of centering and can cover an extended range of speeds without presenting means external to the helicopter fuselage.
The invention proposes a device for measuring, especially advantageous close to the stationary flight mode with reference to the ground, the velocity of a helicopter with reference to the air, the helicopter having at least one lift rotor and a system for cyclic command of the rotor pitch, maneuvered by means of a piloting control column, the device comprising two substantially identical measurement systems associated with a double indicator and corresponding respectively to the movement of the helicopter along its longitudinal and transverse axes.
According to the invention, the above device is characterized in that each of the measurement systems comprises a first detector of the position of the cyclic rotor pitch command along the axis in question, and a second detector of the component of acceleration along the same axis, computing means being provided, assoicated with the said detectors, to effect, with reference to each of the said axes, the integration of the algebraic sum of the values measured by two corresponding detectors.
In the theory of helicopter flight stability, it is shown that the amplitude of displacement of the cyclic control of the rotor along one of the axes of reference, whether longitudinal or transverse, is the sum of a value that is in proportion to the fuselage trim in the vertical plane referred to the axis in question, and another value that is in proporation to the velocity component along the same axis, for a substantially constant rotor running operation. Because of this proposition, the device of the invention makes it possible to obtain the velocity value conveniently, making use only of means internal to the helicopter fuselage, as explained below.
In a preferred embodiment of the invention, for a helicopter equipped with servo controls of cyclic variation of the rotor pitch, the two detectors of position of the rotor controls are mounted between the latter and the servo controls, each including means for adjustment with reference to a zero position. Preferably each acceleration detector has a pendular level and reset means to adjust the pendular level with respect to a reference axis, each of said detectors including a measurement potentiometer.
Advantageously, coupling means are likewise provided between the detectors of position of the rotor command of each measurement system on the one hand, and the output of the computing means of the other system, to compensate variation of the position of the rotor command assocaited with the respective measurement systems under the influence of variation of the velocity component measured by the other system.
As explained below, the arrangements that have just been indicated make it possible to develop a sensitive accurate measurement device, in a fairly wide range of speeds, on helicopters of considerable weight.
Other features and advantages of the invention will become evident from the description of embodiments of the invention, presented as non limitative examples, with reference to the attached drawings.